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How can I improve the solar water pump in my bird bath? Answered

I have a small electric water pump in my bird bath, powered by a small solar panel about 4.5" square. According to the literature that came with the pump, it runs on 7V/1.05W of power. The panel battery is 7V/1.2W.

In practice, this means that the pump runs continuously when the panel is in direct sunlight (about 5 hours a day) and sporadically when it's in the shade.

I would like the pump to run continuously more often. I'm guessing that means I need a storage battery with higher capacity and/or a larger panel. Harbor Freight sells this item, which is 24VDC/1.5W. http://www.harborfreight.com/15-watt-solar-battery-charger-68692.html

What kind of battery would I need, and do I need a regulator as part of the system as well?

Discussions

I suggest using some test equipment (like voltmeter, ammeter, lab bench style DC power supply if you've got one) to measure the power being used by your bird bath pump. For something like this, it is a little naive to just read the numbers they printed on the box, and assume they mean something.

The pump will probably run over a wide range of input voltage. I mean supposing you measured the current draw of the pump under mechanical load (i.e while it is pushing on water), at several different voltages. Then you could make a graph, of I versus V, and call it a I-V curve.

A curve like this can be imagined to exist for the solar panel too. For that test, you measure voltage on the panel, as it changes depending on the current draw of different test loads. For this test you want the panel to be under essentially constant illumination. I mean what you get is an I-V curve for that particular level of light power falling on the panel, e.g. an I-V curve for "full sun". For lesser levels of illumination, you get sort of set of smaller curves. Conveniently, they can all fit on the same graph, because the smaller curves sort of nest inside the bigger ones.

I mentioned there was a believability problem for the numbers printed on the pump, and a similar, or worse, problem exists for the numbers used to sell photovoltaic (PV) panels. There is sort of tendency on the part of the ad writers to quote the biggest, baddest, most impressive sounding numbers they can find. Usually that means quoting, open-circuit voltage (Voc), short-circuit current (Isc), and both of those under full sun. Then the truly bold will multiply those numbers together to give you a (dishonest) quote for power.

I know probably wanted someone on this forum to just, like, do the math for you, and recommend something to buy, and then see if that worked...

However, I think you will truly have a better handle on this if you actually do the measurements, and the math yourself.

Also, I think for something like this, there is a lot of freedom to experiment, and little danger of overpowering the pump just by hooking new stuff up to it to see what happens. I mean if the pump is immersed in water, that should make it harder to melt by way of overpowering it. ;-)

By the way, I cannot believe there is actually a storage battery as part of this solar bird bath toy. Did I read that part right?

Here's the actual item that I purchased. I've had it for about a year and a half. http://www.dx.com/en/p/solar-powered-panel-water-pump-pond-fountain-pool-black-71720?tc=USD&gclid=CjwKEAjwkcWrBRDg5u6SuPS11C0SJAChLLAH1S82_5Hm8Oew_gaYncbrtaiHUCPTBhkmk7G4aaNJcxoC6nnw_wcB#.VXH-689VjXs

The panel has a small box protruding from the back. I believe it contains some kind of rechargeable cell(s). I may be wrong, but there's no good way to get into it without breaking the whole thing. It's sealed with epoxy.